1. Field of the Invention
The present invention relates to a process for preparing para-hydroxybenzoic acid, in particular, a process by which para-hydroxybenzoic acid can be obtained not only in high yield but also in high selectivity since the generation of by-product salicylic acid can be suppressed.
2. Prior Art and Problems
Para-hydroxybenzoic acid finds a wide variety of applications as starting material for polymeric materials. Particularly in these days, it draws attention as starting material for liquid crystalline polyesters having high strength and high heat resistance. Also alkyl esters of para-hydroxybenzoic acid are useful as antimolds of cosmetics and industrial materials.
Para-hydroxybenzoic acid is basically produced by reacting an alkali salt of phenol with carbon dioxide. The processes for producing para-hydroxybenzoic acid are roughly divided into the Kolbe-Schmitt process in which the reaction between an alkali salt of phenol and carbon dioxide is carried out at elevated temperatures and pressures, and the Kolbe process in which the same reaction is performed at elevated temperatures under atmospheric pressure. It has been known for many years that whichever process is adopted, salicylic acid is generated as the main product if sodium phenolate is used as the starting material whereas para-hydroxybenzoic acid is generated if potassium phenolate is used.
The Kolbe-process involving reaction at atmospheric pressure has several advantages over the Kolbe-Schmitt process employing superatmospheric pressure, such as simplicity of apparatus, low equipment cost, ease of operations and control, and adaptability to continuous reaction. Nevertheless, the Kolbe-Schmitt process in which the reaction is carried out at elevated temperatures and pressures is currently used to produce para-hydroxybenzoic acid on an industrial scale, because the product yield by the Kolbe-Schmitt process is superior to that by the Kolbe process in which the same reaction is performed at elevated temperatures under atmospheric pressure.
Even by the Kolbe-Schmitt process, only about 50% of the yield can be achieved in most cases of the production of para-hydroxybenzoic acid, while a high yield of around 90% can be obtained in the case of the production of salicylic acid.
Recent studies on the Kolbe-Schmitt process have revealed that in the solid-gas phase reaction, not only is the efficiency of stirring operations low but also carbon dioxide is prevented from making effective contact with potassium phenolate. Further, heat transfer tends to become nonuniform, causing such shortcomings as the generation of by-product salicylic acid in increased amounts and the prolonged reaction time which takes as many as 4-7 hours. The by-product salicylic acid is an isomer of the desired para-hydroxybenzoic acid, the two compounds are similar in solubility and other physical properties. Hence, as the amount of the by-product salicylic acid generated increases, it becomes increasingly difficult to achieve efficient separation from para-hydroxybenzoic acid without reducing the purity and yield of the latter. All of these phenomena are deleterious in that the para-hydroxybenzoic acid obtained is not suitable for use in its intended applications.
With a view to eliminating the above-described shortcomings of the solid-gas phase reaction involved in the Kolbe-Schmitt process, the use of various inert reaction media has been proposed, as described in many patcnts including for example, Japanese Patent Publication No. 26612/1968, Japanese Patent Application Kokai No. 164751/1984, Japanese Patent Publication No. 9453/1971, etc. The method described in Japanese Patent Publication No. 9453/1971 is characterized by performing reaction in an aprotic polar solvent in the presence of an alkali metal or an alcoholate thereof. The yield of the para-hydroxybenzoic acid that can be obtained by this method is as high as 94% but at the same time, the amount of salicylic acid generated as a by-product is no smaller than 6%. In addition, alkali metals are not only expensive but also very hazardous since they will combine with water or methanol in an explosive way. Hence, alkali metals are not suitable for use in large-scale production. Even if one chooses to use alcoholates of alkali metals, they must be synthesized from alkali metals and alcohols, thus involving the same shortcomings as in the case using of alkali metals. Further, the lower alcohols that form as by-products will inhibit the progress of reaction and this necessitates performing the reaction with such alcohols being removed from the system.
Inert reaction media other than alkali metals and alcoholates thereof are also used in the prior art and it has been reported that the amount of salicylic acid generated as a by-product is a little less than 8% on the higher-content side and ranges from "not detected" to a little less than 2% on the lower-content side. However, even with these conventional methods, the yield of para-hydroxybenzoic acid that can be attained is generally about 50% and the exceptionally good result that has ever been reported is 77.3% (as described in Japanese Patent Application Kokai No. 164751/1984). Thus, up to the present day, there have been no processes in the art by which para-hydroxybenzoic acid can be produced at higher yields while the generation of the by-product salicylic acid can be suppressed to lower levels.
Prior to the application for this patent, the inventors of the present invention have applied for a patent (PCT/JP88/01051) with regard to a process for producing para-hydroxybenzoic acid in high yield by reacting an alkali phenolate with carbon dioxide in the presence of a substituted alkali phenolate (alkali metal; potassium, rubidium or cesium). In this prior application, however, a subject for repressing by-production of salicylic acid (that is, improvement of selectivity for the production of para-hydroxybenzoic acid) was remained unsolved. Therefore, the present inventors have performed detailed studies using potassium as an easily available alkali metal in order to solve this problem and have achieved the present invention.
It is, therefore, an object of the present invention to provide a process by which para-hydroxybenzoic acid having utility in a broad range of applications as the starting material for the production of various polymeric materials can be prepared not only in high yield but also with high selectivity and purity since the generation of the by-product salicylic acid can be suppressed to a lower level.